Sensor elements for determining oxygen content are composed of ceramic foils made of, for example, stabilized ZrO.sub.2, which are imprinted with electrodes having electrode leads and a heating device having a heating conductor and heating conductor leads, and which, after being laminated together and then sintered, yield a planar sensor element. In the case of electrochemical sensors, the heating device is embedded between two electrically insulating layers, for example, made of Al.sub.2 O.sub.3, which, in turn, are arranged, in each case, with regard to the adjoining oxygen-ion-conducting ceramic sheets.
In electrochemical sensor elements that operate using a reference gas, an electrode is disposed in a reference gas channel, which extends, for example, in the longitudinal direction of the sensor element and is in contact with ambient air. This reference gas channel runs in a plane of the layer between the reference electrode and the heating device.
In those areas in which the heating conductor spans the reference gas channel, there is poor heat dissipation for the heat generated, so that in this area it is possible that the heating conductor may overheat. In addition, the poor heat conduction in the area of the reference gas channel means that it is only possible to heat the sensor elements to their operating temperature using increased heating energy or after a correspondingly long warm-up period.
German Patent No. 196 09 323 describes that improved heat conduction may be achieved by arranging the heat conductor of the heating device outside the projection of the reference gas channel. For this purpose, the reference gas channel has been designed in two parts, in the area of the electrodes and the heat conductor.